>perl -wMstrict -le
"for my $line ('', 'X', 'Y') {
   for my $locus_acc_no ('', 'X', 'Y') {
     if ($line =~ MatchLAC($locus_acc_no)) {
       print qq{   match: '$line' =~ MatchLAC('$locus_acc_no')};
       }
     else {
       print qq{NO match: '$line' =~ MatchLAC('$locus_acc_no')};
       }
     }
   }
 ;;
 sub MatchLAC { return $_[0] =~ /^X$/; }
"
   match: '' =~ MatchLAC('')
NO match: '' =~ MatchLAC('X')
   match: '' =~ MatchLAC('Y')
   match: 'X' =~ MatchLAC('')
NO match: 'X' =~ MatchLAC('X')
   match: 'X' =~ MatchLAC('Y')
   match: 'Y' =~ MatchLAC('')
NO match: 'Y' =~ MatchLAC('X')
   match: 'Y' =~ MatchLAC('Y')