use List::Util qw( reduce );

sub bin2dec {
	my ( $str ) = @_;
	reduce { mul2( $a, $b ) } "", split //, $str;
}

##</code><code>##

sub mul2 {
	my ( $str, $have_overflow ) = @_;

	my $ret = join '', reverse map {
		my $c = $_ * 2 + ( $have_overflow ? 1 : 0 );
		$have_overflow = ( $c >= 10 );
		( $c % 10 );
	} reverse split //, $str;

	( $have_overflow ? 1 : '' ) . $ret;
}

##</code><code>##

sub mul2 {
	my ( $str, $have_overflow ) = @_;
	join '', reverse map {
		my $c = $have_overflow ? 1 : 0;
		$c += $_ * 2 if length;
		$have_overflow = ( $c >= 10 );
		# return a zero char only if not at top of string
		( $c % 10 ) or ( length() ? 0 : '' );
	} reverse '', split //, $str;
}