More importantly, did you know you can have the substitute expression treated as Perl code using the e modifier?

for (@ARGV) {
    s!  (\d{2})/
        (\d{2})/
        (\d{2})
    !
        my $y = ( $3 > 55 ? '19' : '20' ) . $3;
        "$y/$1/$2"
    !xeg;

    print "$_\n";
}
[download]

More tips:

• gt is to compare strings. > is to compare numbers.
• Don't use the o modifier. It only adds disadvantages (no advantages) nowadays.
• Use local our $var; instead of my $var; for vars outside of regexps used in (?{}) and (??{}). It doesn't always make a difference, but it'll save yourself from subtle errors.
• Name a variable holding a year something other than $d.

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