Switching my for our works fine for me with identical setup: AS 5.8.4/XP. Though I have AS 510 and there was an earlier version (AS508?) which I saw another problem that the move to 510 fixed. Maybe this is another?

use strict;
use warnings;

while ( <DATA> ) {
    chomp;
    print bin_to_dec($_), "\n";
}

sub bin_to_dec {
    my $bits = shift;
    our( $power, $magnitude, $num );
    die "$bits is not a pure bit string.\n" if $bits =~ m/[^10]/;
    if (
        $bits =~ m/
                    (?{
                      $power = length($_) - 1;
                      $num = 0;
                    })
                    (?:
                      ([10])
                      (?{
                        $magnitude = 2 ** $power;
                        $^N eq '1' and $num += $magnitude;
                        $power--;
                      })
                    )+
                  /x
    ) {
        return $num;
    } else {
        die "Unable to resolve bits: $bits.\n";
    }
}

__DATA__
00000000
00000011
00000111
11100000
__OUTPUT__
P:\test>junk
0
3
7
224
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