It won't work because map and grep both localise $_: inside the braces, $_ is aliased to each element of @sybfile in turn. But, grep also aliases $_ to each element of @ingfile, which hides the former $_.

In fact, you end up doing $_ =~ /$_/, which is pretty much always true. You have to save the map's $_ into some temporary variable:

    print STDERR "DEBUG YYY\n" unless map {
        my $tmp = $_;
        grep /\Q$tmp\E/, @ingfile;
    } @sybfile;
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HTH

--bwana147