Tristate... you said the magic word!

my $test = return_from_function();

if (not defined $test) {
  # do this if evaluates to undefined
} elsif (!$test) {
  # do this if evaluates to zero equivalent
} elsif ($test) {
  # do this if evaluates to non-zero equivalent
}
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If you really wanted, you could throw in a $test eq '' in there too, but I've found that to be gratuitous code abuse.