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

multi sub trim($str is rw) {
    # modify in place
}

##</code><code>##

subtest 'trim' => sub {
    is("leading", trim("   leading"), "trim: leading");

##</code><code>##

$ 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.