Part 1 gives a general overview of the meaning of operators.
Part 3 gives an overview of complex regular expressions.

A basic match

if ($string =~ /abc/){#code goes here}

The code is executed if the string contains abc anywhere in the varible $string
The opposite of this is:

if ($string !~ /abc/){#code goes here}

Test for abc at the begining

if ($string =~ /^abc/){#code goes here}

or end of a string

if ($string =~ /abc$/){#code goes here}

You can tell the match operator to ignore case with an i at the end:

if ($string =~ /abc/i){#code goes here}

matches ABC -- abc --AbC -- ABc and so on..
-or-
This uses the translate operator to make sure all letters are lower case:

$input =~ tr/A-Z/a-z/; 

Okay, lets see if you can figure out what this one does:

if ($count =~ /\n$/) {
   chop($count);
}

Here's an explanation. Chop removes the last character in a string, not just a line feed. This tests the string for a line feed before removing it. It is similar to chomp, which extracts all line feeds from a variable.

This regular expression leaves only numbers in $input

$input=~tr/0-9//cd;

And this is here because I use it all the time and can just come here and swipe it into my code:

if ($ENV{'HTTP_REFERER'} ne "" && $ENV{'HTTP_REFERER'} !~ /goldenink.com/){}

This is the standard code to clean out the trash that a web server throws into a URL if you are handling input from a "GET" action.

sub HtmlOut {

        # Decode the form encoding on the variables.          #
        $formfield =~ tr/+/ /;
        $formfield =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;

}

What it does:
The first line changes all '+' signs to blanks.
The second lines substitutes standard characters for the % representations.
Let's try one of the predefined escape sequences to split a string on the "whitespace";

   @words = split(/\s+/, $string);

Predefined sequences
\d	Numeric [0-9]
\D	Non-numeric [^0-9]
\w	Alphameric [a-zA-Z0-9]
\W	Non alphameric [^a-zA-Z0-9]
\s	Whitespace [ \t\n\r\f]
\S	Non whitespace [^ \t\n\r\f]

Part 3 gives an overview of complex regular expressions.

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