What if get two paired variables, and want to check that both are initialized, or both are empty or null strings?

If '0' isn't a valid value

    if( ! $one == ! $two ) {
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If '0' is a valid value but '' is not

    if( ! length $one == ! length $two ) {
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If '' is also a valid value

    if( defined $one == defined $two ) {
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That middle case isn't sufficient on rather old versions of Perl because it used to be that length(undef) was '0' with a warning. Now length(undef) is undef so you'll only get the warning if you treat the length as a number.

Some people replace '!' with '!!' because they use '!!' as an idiom for "I just care about whether it is true or not" or "convert to Boolean".

[ Update: I've seen people use something like

    unless( $one xor $two ) {
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But I find it way too easy to end up making mistakes when you start using the ultra-low-precendence Boolean operators for things other than flow control (I've found quite a few such mistakes made by some of the most experienced Perl programmers I've worked with), so I don't really like that approach. (I also don't like 'unless' but I won't go into that here.) ]