use warnings;
use strict;

my $bin = '1001100010100001';
print add_space($bin), "\n";

sub add_space {
    # Add space separator
    # Code adapted from perlfaq5: How can I output my numbers with commas added?
    local $_ = shift;
    1 while s/^([01]+)([01]{4})/$1 $2/;
    return $_;
}

__END__

1001 1000 1010 0001