• use index to find the start of $s2 in $s1
• add the length to the start position
• apply substr

and please try to format your posts

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
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You should read up on index, length and substr:

#!/usr/bin/perl
use strict;
use warnings;

# Example strings
my $string1 = "ABCDEFderp12345";
my $string2 = "derp";

# Find position of string2 within string1
my $strpos = index($string1, $string2);

# If $strpos is greater than zero (see perldoc index)
if ( $strpos >= 0 ){

    # Print remainder of the string
    # See perldoc length, perldoc substr
    print substr( $string1, ( $strpos + length($string2) ) );
}else{
    print "$string2 does not occur within $string1";
}
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In the above basic example above we find out if $string2 exists within $string1. If $string2 does not exist within $string1 then index will return -1, otherwise it'll return the position where $string2 occurs. substr can be used to get the text from $string1 after $string2, but we need to add the length of $string2 to the position returned by index as it's offset, because you don't want to include $string2 in what gets printed.

Further recommended reading:

• perlintro
• http://learn.perl.org
• http://perldoc.perl.org
• The tutorials section of this site, including PerlMonks for the Absolute Beginner

Update: changed gt 0 to >= 0. I wasn't quite awake when I posted. Thanks AnomalousMonk for pointing this out.

Update 2: Slight rewording for clarity.

With

perldoc -f index
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use strict;
use warnings;
my $string1 = "ABCDEFderp12345";
my $string2 = "derp";
if ($string1 =~ m/$string2(.*)$/  ){
    print $1, "\n";
}else{
    print "$string2 does not occur within $string1";
}
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Better to meta-quote  $s2 to avoid strange effects from regex metacharacters in it ( //p regex modifier available with 5.10+):

>perl -wMstrict -le
"my $s1 = 'foo not *UN*known but bar is';
 my $s2 = '*UN*known';
 ;;
 die qq{'$s2' not in '$s1'} unless $s1 =~ m{ \Q$s2\E }xmsp;
 print qq{all after: '${^POSTMATCH}'};
"
all after: ' but bar is'
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Start by trying and showing us what you have tried :) See perlintro, substr