I used this script for testing:

#!/usr/bin/perl -w

use strict;

my $pattern = "*.old";
# unlink <$pattern>;
unlink <*.old>;
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With use strict in place, uncommenting the first line reveals that Perl thinks $pattern holds a symbolic reference, at least in the way you've used it. That's what you meant it to do, but the interpreter doesn't do two levels of interpretation.

The first level of interpretation is looking at what is in $pattern -- *.old in this case. It stops there, instead of interpreting THAT as a glob.

The second example works because it can only be interpreted as a glob. You're just one level trickier than the Perl interpreter wants to be in this case.