Just a matter of subtraction then to go back a specified number of characters from the end of a string.

#!perl -w
use strict;

my $str = "me like cookies";
my $slen = length($str);

my $howfarback = 7;

#Pick from $howfarback position back for substring
my $newstr=substr($str, $slen - $howfarback, $howfarback);

print "$newstr \n";
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nandeya