This will not work:

print "@a\n" if List::Util::first { 0 == $_ } @a

When using first as a condition, it's best to wrap it in a defined() to specifically check for the 'not found' case:

# this always works
print "@a\n" if defined(List::Util::first { defined($_) && 0 == $_ } @
+a);
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OTOH, I agree that first clobbers the predicate when checking for undef -- i.e. we can't tell if we found undef:

# doesn't work
print "@a\n" if defined(List::Util::first { !defined($_) } @a);
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