This is a good time to start always using the three parameter version of open and lexical file handles:

open my $file4In, '<', $fileName or die "Could not open $filename: $!\
+n";
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Appending In or Out to your file handles helps as a sanity and consistency check.

You get even more linty goodness if you always use strictures (use strict; use warnings;).

Note too that showing why an open failed often is at least as useful as showing which open failed.