You should probably pick up a book, the O'Reilly books are generally really good and I believe "Learning Perl" is the intro book. If you're learning on Linux and you have the perl docs installed you probably have the perl manages at your disposal, so you could type 'man perldata' to read about the perl data types. For arrays and hashes it says this: Normal arrays are ordered lists of scalars indexed by number, starting with 0. Hashes are unordered collections of scalar values indexed by their associated string key.
Environment variables are just variables used by the shell, in bash you can type 'set' to see all the variables available at the moment.
STDIN is standard input, generally the terminal so you read from STDIN when you want to ask the user something. But STDIN and STDOUT can be redirected, so you could type 'cat somefile | myperlscript.pl' and now STDIN is the file somefile instead of user input.
Obviously not all the docs available are going to be relevant, but the perl man pages on my system are:
perl (1) - Practical Extraction and Report Language
perl5004delta (1) - what's new for perl5.004
perl5005delta (1) - what's new for perl5.005
perl561delta (1) - what's new for perl v5.6.x
perl56delta (1) - what's new for perl v5.6.0
perl570delta (1) - what's new for perl v5.7.0
perl571delta (1) - what's new for perl v5.7.1
perl572delta (1) - what's new for perl v5.7.2
perl573delta (1) - what's new for perl v5.7.3
perl581delta (1) - what is new for perl v5.8.1
perl582delta (1) - what is new for perl v5.8.2
perl583delta (1) - what is new for perl v5.8.3
perl584delta (1) - what is new for perl v5.8.4
perl58delta (1) - what is new for perl v5.8.0
perlXStut (1) [perlxstut] - Tutorial for writing XSUBs
perlaix (1) - Perl version 5 on IBM Unix (AIX) systems
perlamiga (1) - Perl under Amiga OS
perlapi (1) - autogenerated documentation for the perl public API
perlapio (1) - perl's IO abstraction interface.
perlapollo (1) - Perl version 5 on Apollo DomainOS
perlartistic (1) - the Perl Artistic License
perlbeos (1) - Perl version 5 on BeOS
perlbook (1) - Perl book information
perlboot (1) - Beginner's Object-Oriented Tutorial
perlbot (1) - Bag'o Object Tricks (the BOT)
perlbs2000 (1) - building and installing Perl for BS2000.
perlbug (1) - how to submit bug reports on Perl
perlcall (1) - Perl calling conventions from C
perlcc (1) - generate executables from Perl programs
perlce (1) - Perl for WinCE
perlcheat (1) - Perl 5 Cheat Sheet
perlclib (1) - Internal replacements for standard C library functions
perlcn (1) - ¼òÌåÖÐÎÄ Perl Ö¸ÄÏ
perlcompile (1) - Introduction to the Perl Compiler-Translator
perlcygwin (1) - Perl for Cygwin
perldata (1) - Perl data types
perldbmfilter (1) - Perl DBM Filters
perldebguts (1) - Guts of Perl debugging
perldebtut (1) - Perl debugging tutorial
perldebug (1) - Perl debugging
perldelta (1) - what is new for perl v5.8.4
perldgux (1) - Perl under DG/UX.
perldiag (1) - various Perl diagnostics.
perldoc (1) - Look up Perl documentation in Pod format.
perldos (1) - Perl under DOS, W31, W95.
perldsc (1) - Perl Data Structures Cookbook
perlebcdic (1) - Considerations for running Perl on EBCDIC platforms
perlembed (1) - how to embed perl in your C program
perlepoc (1) - Perl for EPOC
perlfaq (1) - frequently asked questions about Perl ($Date: 2003/01/31 17:37:17 $)
perlfaq1 (1) - General Questions About Perl ($Revision: 1.14 $, $Date: 2003/11/23 08:02:29 $)
perlfaq2 (1) - Obtaining and Learning about Perl ($Revision: 1.25 $, $Date: 2003/10/16 04:57:38 $)
perlfaq3 (1) - Programming Tools ($Revision: 1.37 $, $Date: 2003/11/24 19:55:50 $)
perlfaq4 (1) - Data Manipulation ($Revision: 1.54 $, $Date: 2003/11/30 00:50:08 $)
perlfaq5 (1) - Files and Formats ($Revision: 1.30 $, $Date: 2003/11/23 08:07:46 $)
perlfaq6 (1) - Regular Expressions ($Revision: 1.20 $, $Date: 2003/01/03 20:05:28 $)
perlfaq7 (1) - General Perl Language Issues ($Revision: 1.15 $, $Date: 2003/07/24 02:17:21 $)
perlfaq8 (1) - System Interaction ($Revision: 1.17 $, $Date: 2003/01/26 17:44:04 $)
perlfaq9 (1) - Networking ($Revision: 1.15 $, $Date: 2003/01/31 17:36:57 $)
perlfilter (1) - Source Filters
perlfork (1) - Perl's fork() emulation
perlform (1) - Perl formats
perlfreebsd (1) - Perl version 5 on FreeBSD systems
perlfunc (1) - Perl builtin functions
perlgpl (1) - the GNU General Public License, version 2
perlguts (1) - Introduction to the Perl API
perlhack (1) - How to hack at the Perl internals
perlhist (1) - the Perl history records
perlhpux (1) - Perl version 5 on Hewlett-Packard Unix (HP-UX) systems
perlhurd (1) - Perl version 5 on Hurd
perlindex (1p) - index and query perl manual pages
perlintern (1) - autogenerated documentation of purely internal Perl functions
perlintro (1) - - a brief introduction and overview of Perl
perliol (1) - C API for Perl's implementation of IO in Layers.
perlipc (1) - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores)
perlirix (1) - Perl version 5 on Irix systems
perlivp (1) - Perl Installation Verification Procedure
perljp (1) - ÆüËܸì Perl ¥¬¥¤¥É
perlko (1) - Perl°ú Çѱ¹¾î ÀÎÄÚµù
perllexwarn (1) - Perl Lexical Warnings
perllocale (1) - Perl locale handling (internationalization and localization)
perllol (1) - Manipulating Arrays of Arrays in Perl
perlmachten (1) - Perl version 5 on Power MachTen systems
perlmacos (1) - Perl under Mac OS (Classic)
perlmacosx (1) - Perl under Mac OS X
perlmint (1) - Perl version 5 on Atari MiNT
perlmod (1) - Perl modules (packages and symbol tables)
perlmodinstall (1) - Installing CPAN Modules
perlmodlib (1) - constructing new Perl modules and finding existing ones
perlmodstyle (1) - Perl module style guide
perlmpeix (1) - Perl/iX for HP e3000 MPE
perlnetware (1) - Perl for NetWare
perlnewmod (1) - preparing a new module for distribution
perlnumber (1) - semantics of numbers and numeric operations in Perl
perlobj (1) - Perl objects
perlop (1) - Perl operators and precedence
perlopentut (1) - tutorial on opening things in Perl
perlos2 (1) - Perl under OS/2, DOS, Win0.3*, Win0.95 and WinNT.
perlos390 (1) - building and installing Perl for OS/390 and z/OS
perlos400 (1) - Perl version 5 on OS/400
perlothrtut (1) - old tutorial on threads in Perl
perlpacktut (1) - tutorial on *(C`pack*(C' and *(C`unpack*(C'
perlplan9 (1) - Plan 9-specific documentation for Perl
perlpod (1) - the Plain Old Documentation format
perlpodspec (1) - Plain Old Documentation: format specification and notes
perlport (1) - Writing portable Perl
perlqnx (1) - Perl version 5 on QNX
perlre (1) - Perl regular expressions
perlref (1) - Perl references and nested data structures
perlreftut (1) - Mark's very short tutorial about references
perlrequick (1) - Perl regular expressions quick start
perlreref (1) - Perl Regular Expressions Reference
perlretut (1) - Perl regular expressions tutorial
perlrun (1) - how to execute the Perl interpreter
perlsec (1) - Perl security
perlsolaris (1) - Perl version 5 on Solaris systems
perlstyle (1) - Perl style guide
perlsub (1) - Perl subroutines
perlsyn (1) - Perl syntax
perlthrtut (1) - tutorial on threads in Perl
perltie (1) - how to hide an object class in a simple variable
perltoc (1) - perl documentation table of contents
perltodo (1) - Perl TO-DO List
perltooc (1) - Tom's OO Tutorial for Class Data in Perl
perltoot (1) - Tom's object-oriented tutorial for perl
perltrap (1) - Perl traps for the unwary
perltru64 (1) - Perl version 5 on Tru64 (formerly known as Digital UNIX formerly known as DEC OSF/1) systems
perltw (1) - ¥¿Å餤¤å Perl «ü«n
perlunicode (1) - Unicode support in Perl
perluniintro (1) - Perl Unicode introduction
perlutil (1) - utilities packaged with the Perl distribution
perluts (1) - Perl under UTS
perlvar (1) - Perl predefined variables
perlvmesa (1) - building and installing Perl for VM/ESA.
perlvms (1) - VMS-specific documentation for Perl
perlvos (1) - Perl for Stratus VOS
perlwin32 (1) - Perl under Windows
perlxs (1) - XS language reference manual
perlxstut (1) - Tutorial for writing XSUBs