It's because the value of $baseball{mets} is not defined at the point you're trying to use it.

Note how the warning goes away with something like:

my $yanks = $baseball{yankees} || "";
my $mets  = $baseball{mets}    || "";
if ( $baseball{yankees} = $yanks.$mets )  {
    foreach $_ (keys %baseball) {
        print "$_ => $baseball{$_}\n";
    }
}
[download]

You could also do it this way:

if ( $baseball{yankees}=($baseball{yankees} || ""). ($baseball{mets} |
+| "") )  {
    foreach $_ (keys %baseball) {
        print "$_ => $baseball{$_}\n";
    }
}
[download]

This is all assuming, of course, that you really mean to do the assignment $baseball{yankees}=$baseball{yankees}.$mets.

If instead you are doing a comparison, you'd probably want to use eq (for string comparisons) rather than == (for numerical comparisons).