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
$
I hope this is useful.
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