Only scenario that comes to my mind is when author of class that implements object $x overrides method 'can'. But if such an author would do this, he is supposed to know what he is doing, as 'can' is a well known method together with 'isa'.

When $x isn't a blessed reference!

-- Randal L. Schwartz, Perl hacker

Or when it doesn't contain a class name:

package Test;

sub foo {};

package main;

if (Test->can('foo')) {
    print "Hardcoded can works!\n";
}

my $t = 'Test';
if ($t->can('foo')) {
    print "Variable can works!\n";
}
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Now you or I wouldn't usually let that pass in a code review, which is why I go the extra step of invoking UNIVERSAL::can(). But we both know one dirty little secret of OO Perl, that it's the class name that's important, not the fact that something has been blessed.

When can't you call $x->can('foo') safely? When $x can't can(), of course!

for $x ('main', undef, \1) {

  if (UNIVERSAL::can($x, 'can')) {

    # ok

    print "can can\n";
    $x->can('foo');
    print "\n";

  } else {

    # error: Can't call method "can" on undefined value/unblessed refe
+rence ...

    print "can't can\n";
    eval { $x->can('foo') };
    print "$@\n";

  }

}
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Clarification: The else part wouldn't generate an error for 'main' but would/does for the other two (well, it wasn't clear to me at first but I think that is more my problem than chipmunk's).

One case you didn't mention is "strings that don't start with a \w character":

$x= '*fred';
print $x->can('can');

Can't call method "can" without a package or object reference
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and, yes, it is (currently, at least) only the first character that matters for this test.

BTW, these errors are fatal (which is why chipmunk used eval, otherwise the loop would not have continued).

Updated many times while I tried to make some sense.

        - tye (but my friends call me "Tye")