Well, lots of ways spring to mind. I prefer to think about such problems in terms of how the caller will have to deal with things. I was going to list several examples but once I hit this one I threw the other ones away:

if(  ! check_foo("foo",\$foo) )  ) {
    warn "No foo present\n";
} else {
    do_stuff( $foo );
}
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        - tye (but my friends call me "Tye")