or, to avoid modifying $str and be rid of trailing spaces

my $tmp = $str;
$tmp =~ s/((?:\s*\d+){20})/$1\n/g;
print OUT "$tmp\n";
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update: get rid of leading spaces

my $tmp = $str;
$tmp =~ s/\s*((?:\s*\d+){20})/$1\n/g;
print OUT "$tmp\n";
[download]