...
 if ( $str =~ / / ) {  
    print "no underscore\n";  
 }
 else {
    print "with underscore\n";
 }
 ...

##</code><code>##

  ...
  my $str = $ARGV[0] || '|78187980|ref|NM_0';          # original string

  my $cnt = $str =~ tr/_//;                            # count the number of underscores

  print 'with ' . ($cnt || 'no') . " underscore(s)\n"; # print result depending on count
  ...