use strict;
use warnings;

print bin_to_dec('1101100');

sub bin_to_dec {
    my $bits = shift;
    my( $power, $magnitude, $num );
    die "$bits is not a pure bit string.\n" if $bits =~ m/[^10]/;
    if (
        $bits =~ m/
                    (?{
                      $power = length($_) - 1;
                    })
                    (?:
                      ([10])
                      (?{
                        $magnitude = 2 ** $power;
                        $^N eq '1' and $num += $magnitude;
                        $power--;
                      })
                    )+
                  /x 
    ) {
        return $num;
    } else {
        die "Unable to resolve bits: $bits.\n";
    }
}