while($string=~ m/reg(ex)/)
{
    $string=~ s/$1/ister/;
}
[download]

That frightens me a bit. Your substitution replaces the first occurance "ex" with "ister" in the string, no matter whether "ex" is part of "regex" or something else:

my $string = "Sometimes there are extra effects you didn't foresee whe
+n using a regex.";

while($string=~ m/reg(ex)/)
{
    $string=~ s/$1/ister/;
}

print $string;
[download]

Output:

Sometimes there are istertra effects you didn't foresee when using a r
+egister.
[download]

Istertra?