$ cat dumps_core.pl 
#!/usr/bin/perl -w

use strict;
my $countSpaces = sub {
        my ($c) = ($1);
        my @spaces = ($c =~ /( )/g);
        return qr{(?!)} if $#spaces > 0; 
        return qr{(?=)};
};

while (<DATA>) {
        #my @spaces = (/( )/g);
        print "$_ ", /(.*)(??{$countSpaces->()})/ ? "has 1 or no spaces\n" : "has more than one space\n";
}


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

$ ./dumps_core.pl 
Segmentation fault (core dumped)