my @sourcefiles = &get_files($source_directory);

You don't need the ampersand in Perl 5 to call a function. Quite the opposite is true: You should avoid the ampersand in Perl 5 when calling functions, as it can introduce nasty, hard-to-debug errors. The ampersand was needed in Perl 4, but that was decades ago.

Read more at Re^2: Merge log files causing Out of Memory (just a note on ampersand).

Alexander