open (DATA,"$input_file") || eval {
  print "Can't open file $input_file: $@";
  exit;
};
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BTW: Are you aware that this is a very strange error handler for an open statement? As best I can figure, it uses an eval expression to print the value of a previous (Update: No! See below.) eval error ($@ - see perlvar; in particular, Error Variables) as the file open error message. Why not just use a 'standard'
    open my $fh, '<', $input_file or die "opening '$input_file': $!";
(see $! in perlvar and Error Variables).

Update: A simple experiment, which I didn't have time to do before, shows that  $@ is emptied upon entry to an eval block:

>perl -wMstrict -le
"eval { die 'zot!' };
 print qq{after first eval: '$@'};
 ;;
 eval { print qq{in second eval: '$@'} };
"
after first eval: 'zot! at -e line 1.
'
in second eval: ''
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In any event,  $@ is not the proper error variable to print after an I/O error.