I can see three errors:

• open(my OUT, ... should be open(my $OUT, ...  Or open(OUT, ....  Same for IN.

• Your file name to open for output is a directory  ('/Users/mgavibrathwaite/Desktop/').  That doesn't work.

• You're printing to STDOUT instead of to the $OUT file handle you tried to open.  That would be print $OUT "foo\n";