Hi leriksen, You were so close to getting it right, if you extend the regexp just a little bit with the match-time interpolation technique.

use strict;
use warnings;

while (<DATA>) {
    chomp;
    print m/(\d{1,20})(??{' ' x (20 - length($1))})/ ?
          "match\n" : "not match\n";
}

__DATA__
"     123451234512345"
"  123451234512345   "
"123451234512345     "
"1234512345          "
"123 451 2345        "
"                    "
[download]

And the output is exactly as expected -

not match
not match
match
match
not match
not match
[download]