You could do it like this:

#!/usr/bin/perl -w

use strict;
use warnings;


# Main program
try_rgb("#ff00ff");


# Subroutines
sub try_rgb {
    my ($rgb) = @_;
    printf "$rgb => (%d, %d, %d)\n", rgb_to_decimal($rgb);
}

sub rgb_to_decimal {
    my ($value) = @_;
    if ($value =~ /^#(..)(..)(..)/) {
        my ($r, $g, $b) = ($1, $2, $3);
        return (hex($r), hex($g), hex($b));
    }
    return 0;
}
[download]

Output is:

#ff00ff => (255, 0, 255)
[download]