use strict;

sub bin2dec {
   my $arg = shift;
   return unpack("N", pack("B32", substr("0" x 32 . $arg, -32 )));
}

my ($binary, $decimal);

print "Please enter a binary number: ";
do { chomp ($binary = <STDIN>) } until (length $binary);
die "That's not a binary number, dude...\n" if ($binary =~ /[^01]/);
$decimal = bin2dec $binary;
print "$binary = $decimal\n";