Balaton: Athanasius has replied:

... I think it’s highly unlikely that this line:
    if ($line =~ ModuleMatching::MatchLAC($locus_acc_no)) {
can be correct. But without knowing what  sub ModuleMatching::MatchLAC is supposed to do, it’s hard to give advice.

I agree that the definition of  ModuleMatching::MatchLAC() given in the OP is most likely some sort of dummy placeholder, but in any event, the given function can be explained as follows (this is for Balaton; I believe Athanasius understands all this quite clearly):

• The function returns the result of a match of the passed argument  $_[0] against a literal regex (i.e., a regex having no interpolations (Update: literal regex: my terminology may be a bit off here));
• The function is called in scalar context imposed by the  =~ operator, so the result of the match within the function is either 1 (successful match) or '' (the empty string; match failed);
• The '' or 1 returned by the function is then converted to a regex (with 1 stringized to '1') and a match is made against  $line. If the match is against  /1/ the result is obvious. If the match is against  // (the empty regex, created from the empty string), the result will come from a match against the regex most recently matched or, if no regex has ever been matched, against the null regex, which matches anything. Straight from the docs: "If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used instead. [...] If no match has previously succeeded, this will (silently) act instead as a genuine empty pattern (which will always match)."

The problematic results of these matches can be illustrated as follows:

>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')
[download]

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