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 com
+mas added?
    local $_ = shift;
    1 while s/^([01]+)([01]{4})/$1 $2/;
    return $_;
}

__END__

1001 1000 1010 0001
[download]

Thank you so much toolic!

Another variation:

$bin = '1001100010100001';;

$bin =~ s[....\K][ ]g;;

print $bin;;
1001 1000 1010 0001
[download]

Thanks for posting this. I have wondered how to do this for a long time.

All the sequences and escapes

Thanks a lot bro! Already got it!

Don't ask what happens if the length of your bin string isn't divisible by 4 ;-)

Haha thanks dude!

my $bin = '1001100010100001';
print join ' ', unpack '(A4)*', $bin;
[download]

Using substr as an alternative to regex substitution and also grouping in fours from the right-hand side in case that is a requirement.

$ perl -Mstrict -Mwarnings -E '
sub ins
{
    my $str = shift;
    my $len = length $str;
    while ( ( $len -= 4 ) > 0 )
    {
        substr $str, $len, 0, q{ };
    }
    return $str;
}

say ins( $_ ) for qw{
   1
   10
   110
   1101
   10010
   101100
   1110101
   10001011
   110100010
   };'
1
10
110
1101
1 0010
10 1100
111 0101
1000 1011
1 1010 0010
$
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I hope this is useful.

Another one starting from right hand side:

my $bin = '101001100010100001';
$bin = reverse join " ", (reverse $bin) =~ /(.{1,4})/g;
print $bin;
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