We'll have to agree to diagree on this one. I find

if ( !$boolean1 and !$boolean2 ) {
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harder to read, and therefore more difficult to comprehend, than

unless ( $boolean1 or $boolean2 ) {
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I make this distinction because the original code was checking *two* conditions, not just one. I will agree that it's a saw-off when comparing

if ( !$boolean ) {
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and

unless ( $boolean ) {
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But for me, if I miss the tiny '!' character, then I have the logic backwards, which is a huge problem. I'm not going to miss the 'unless'.