sub compile($$)
    {
        my ($source_filename, $output_filename) = @_;

        ... # compile source into output
    }

##</code><code>##

    sub compare_modification_times($$)
    {
        my ($file_a, $file_b) = @_;

        ???

    }

    my $src = "wee.foo";
    my $dest = "wee.bar";

    if (compare_modification_times($src, $dest) < 0)
    {
        compile($src, $dest);
    }