That's what I expect too. The following seems to validate that view.

use warnings;
use strict;
use Data::Dump::Streamer;

my $condRes = 1 == 1;

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


sub doCond {
    1 == shift;
}
[download]

Prints:

1 == 1 $VAR1 = 1;
1 == 0 $VAR1 = '';
doCond (1) $VAR1 = 1;
doCond (0) $VAR1 = '';
[download]

You are right that undef is not being returned. False is being returned. False is a dual var. It's 0 when used in numerical context, and '' when used in string context. That's not quite the same as '', as the following shows:

use warnings;

my $false  = !1;
my $nulstr = '';

print("'$false'\n");     # ''
print("'$nulstr'\n");    # '' 

print(0+$false, "\n");   # 0
print(0+$nulstr, "\n");  # 0 with 'Argument "" isn't numeric'
[download]

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