#...

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
}

# ...

##</code><code>##

# ...

use Test::More;
BEGIN {
    if (eval 'use Test::Warn; 1') {
        Test::More->import(tests => 42);
    } else {
        Test::More->import(skip_all => 'Test::Warn is not available');
        # this line won't be reached, see Test::More
    }
}

warning_like { warn "foo" } qr/foo/; 

# ...