1. What does "next unless $_;" do?

It will do the next iteration of the loop (provided the condition is still true) unless $_ is empty. Could also be written as:
if (not $_) {
    next
}

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An empty string, zero, or undef, are all false in Perl.

In reply to Re^2: Duplicate lines with spaces, tabs... by cdarke
in thread Duplicate lines with spaces, tabs... by marto9

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