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if ($DEBUG) { print STDERR "Debug: some value"; }
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...
is pretty clear, but if we re-write that one line, it becomes more of 
+a conversation with the maintainer:

print STDERR "Debug: some value" if $DEBUG;

if ($DEBUG) {
    print STDERR "Debug: some value";
    # some tempory line to debug our debuger ...
    # some other temp line ...
}