I would do it like this. You'll notice that it's similar to a previous solution, but takes the extra step of intentionally failing to match at the end of the string. That way you don't end up with "89_" (where I'm using _ to signify trailing space).

In my example, I'm intentionally printing a '#' at the end of each string to demonstrate that there's no extra whitespace being appended to the end of the string.

use strict;
use warnings;

my @data = ( '12345678', '123456789' );


foreach my $number ( @data ) {
    my $modified = $number;
    $modified =~ s/(\d{2})(?!$)/$1 /g;
    print "$modified#\n";
}
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