How can undef values fall into @triple array?

Anyway here is the fix: 

foreach my $q (@quadruple) {  
    @triple = grep {$_ and $q !~ /$_/} @triple;
}

[download]

In reply to Re^3: searching in array elements substrings contained in another array by ccn
in thread searching in array elements substrings contained in another array by Anonymous Monk

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