If you're using 5.10, you can use the new _ (underscore) prototype to create subroutines that magically use either a passed parameter or $_:

sub display_thing (_) {
    print $_[0]
};

$_='Hello 5.10';
display_thing;
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Another way would be to just wrap display_thing with your own wrapper that handles the decision between $_ and @_:

my $old_display_thing = \&display_thing;
*display_thing = sub {
    if (@_) {
        &$old_display_thing(@_)
    } else {
        &$old_display_thing($_)
    };
};
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But if you're rewriting or wrapping display_thing (and I see no way around rewriting or wrapping it), why not rewrite or wrap it to accept multiple arguments straight away?

sub display_thing {
    for (@_) {
        print "Displaying thing >$_<\n";
    };
};
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