Sometimes, using a newer version of Perl helps. E.g.

use strict;
use warnings;

my $x = "";
my $y;
my $z = "Z";

if ($x =~ /X/ or $y =~ /Y/ or $z =~ /Z/) {
    print "cond true\n"
}
[download]

Perl 5.8.8

$ ./669664.pl
Use of uninitialized value in pattern match (m//) at ./669664.pl line 10.
cond true

Perl 5.10.0

$ ./669664.pl
Use of uninitialized value $y in pattern match (m//) at ./669664.pl line 10.
cond true