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sub display_thing (_) {
    print $_[0]
...

$_='Hello 5.10';
display_thing;

my $old_display_thing = \&display_thing;
*display_thing = sub {
...
        &$old_display_thing($_)
    };
};

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