#...

use Test::More tests => 42;
eval 'use Test::Warn;'; # just ignore if we can't load Test::Warn

#...

SKIP: {
    unless ($INC{'Test/Warn.pm'}) { # test if Test::Warn was loaded
        skip 'Test::Warn is not available', 1;
    }
    warning_like { warn "foo" } qr/foo/; # you may need to put this line in a string eval because warning_like is exported by Test::Warn
}

# ...