Update: ihb pointed this out before I did.

All of the other solutions so far create the anonymous sub inside the calling sub, so they recreate it on every call. A better way that avoids that wasted effort would be

BEGIN {
    my $bar = sub {
        # ...
    };

    sub foo {
        # ...
        $bar->( $baz );
        # ...
        $bar->( $quux );
        # ...
    }
}
[download]

The BEGIN avoids a trap for the unwary: you might otherwise call foo() before the assignment to $bar has run, blowing up the script. If this code is in a module, there is an implicit BEGIN around all of the code, so this might not be necessary (your call), but it's a good defensive habit nonetheless.