Whatever other problems may exist, your open (Ln 9) comes up a little short of a dozen; a few degrees off plumb; or, more directly, wrong:

Your open( my $in, '<', 'non-existing-file' ) || die $!; should be

open( my $in, '<', 'path-to-file-name' ) or  die "Can't open path-to-f
+ile-name, $!";
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(with a [now deprecated, at least by some] bareword and variant quote symbols)

my $datafile;
open (OUT, ">", $datafile ) or die "Can't open $datafile for write, $!
+";
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# for a write