sub warns (&) {
    my $sub = shift;
    my @warns;
    { local $SIG{__WARN__} = sub { push @warns, shift };
        $sub->() }
    return @warns;
}

# example usage
use Test::More tests=>2;
my @w = warns {
   is some_func(), "bar";
};
is grep({/\bfoo\b/} @w), 1;

sub some_func {
    # could be any number of other warnings here
    warn "foo";
    return "bar";
}