You're not doing any error checking. perldoc -f open 
  open(my $fh, '<', "input.txt") or die $!;

and for writing:

  open(my $fh, '>', "output.txt") or die $!;

[download]
Or 
use autodie;

In reply to Re: can't open a file? by Anonymous Monk
in thread can't open a file? by tuvalb

Title:
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