I'm convinced. I just tried out your example, and added a case because I wasn't certain that evaluating a conditional would yield the same as a conditional return. Then, as I've never used Data::Dump::Streamer I also added a case with an explicit undef, just to be certain...

$condRes = condReturn (0);
print 'condReturn (0) '; Dump ($condRes);
$condRes = condReturn (1);
print 'condReturn (1) '; Dump ($condRes);

sub condReturn {
    return "Foobar" if shift == 1;
}

$condRes = mebbeUndef (0);
print 'mebbeUndef (0) '; Dump ($condRes);
$condRes = mebbeUndef (1);
print 'mebbeUndef (1) '; Dump ($condRes);

sub mebbeUndef {
    return undef if shift == 0;
}
[download]

...and got the expected:

condReturn (0) $VAR1 = '';
condReturn (1) $VAR1 = 'Foobar';
mebbeUndef (0) $VAR1 = undef;
mebbeUndef (1) $VAR1 = '';
[download]

--roboticus