...
use strict;
my $countSpaces = sub {
        my $spaces = tr/ //;
        return qr{(?!)} if $spaces > 1; 
        return qr{(?=)};
};

while (<DATA>) {
        chomp;
        print "$_ [", /(.*)(??{$countSpaces->()})/ ? "OK]\n" : "BAD]\n";
}


__DATA__
this line has more than one space
     that_is_5_spaces
This isonespaceonly
therearenospaceshere