Do you really need that <> magic ?

#!perl
use strict;
use File::Copy;
my $extension = '.tmp';

my @files = glob("*.txt");

for my $file (@files) {   
  print "\n--------- $file-----------\n";
  my $backup = $file.$extension;
  rename($file,$backup); # keep as backup
  open IN, '<', $backup or die "$!"; 
  open OUT, '>',$file or die "$!"; # overwrite existing
  while (<IN>) {
    # process lines 
    print OUT $_;
  }
}
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