As chobora points out, == is designed to compare numbers, not strings.
If the intent is to compare string values, "eq" or "ne" is the right
comparison operator.
As LanX points out the right way to test for defined or not defined
is with the "defined" operator.
An expression like say q() eq undef; # 1 will indeed "work",
however this will cause a warning if use warnings; is in effect:
"use of unitialized value in eq".
An idiom that I use often in certain types of code is the use of
the "//=" operator, like this $string //= '';.
Maybe there is something like this:
my ($keyvalue) =~ /someregex (match).../;
# If the match fails, $keyvalue is going to "undefined".
# Rather than test for that with some kind of "if" statement,
$keyvalue //= ''; #does the job
That will assign
$keyvalue to be '' (the null string) if it was "undefined".
That way I can write if ($keyvalue eq 'important'){..}
without a run-time warning if $keyvalue wasn't defined. The
other place that it helps is that I can print $keyvalue without
getting a different kind of warning!
What I'm saying is that sometimes it is a good idea to just
convert potentially undefined strings into '', null strings and
avoid any explicit conditional logic at all to ask about "defined-ness" and just use an obvious string "eq" or "ne" expression. I think the operator showed up in Perl 5.10, but that's been out there for awhile. | 
