(I've written this two days ago and only hit the "preview" button, not "create". What a nice coincidence when replying to an amusing note on stupidity :-)

I don't know the answer either for Perl 5 or Perl 6 (though assigning and check doesn't seem too wrong to me), but in Perl 6 you can use the rw-ness at least as a constraint in multi dispatch (if you think it's a wise idea):

multi sub trim($str) {
    # return modified value
}

multi sub trim($str is rw) {
    # modify in place
}
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(Not implemented yet :/ )

I also have one comment which is totally unrelated.

subtest 'trim' => sub {
    is("leading", trim("   leading"), "trim: leading");
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Test::More says the usage is is($got, $expected, $test_name, so while everything works fine when the passes, you'll get a confusing message when it fails:

$ perl -we 'use Test::More tests => 1; is "leading", " leading", "trim
+: leading"'
1..1
not ok 1 - trim: leading
#   Failed test 'trim: leading'
#   at -e line 1.
#          got: 'leading'
#     expected: ' leading'
# Looks like you failed 1 test of 1.
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