Do you have the following near the start of your script?

use strict;       # essential
use warnings;     # essential

use autodie;      # optional
use diagnostics;  # optional
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The 2 "essentials" catch lots of errors and will save you lots of time. You should always use them!
If you'd had them in your script you'd have got this error message:

-T on closed filehandle $fh at script_name.pl line 2.

The 2 "optionals" can be very useful.
eg use autodie; would have given you this error message:

Can't open 'input2.txt' for reading: 'No such file or directory' at script_name.pl line 1

Documentation:

• strict - Perl pragma to restrict unsafe constructs
• warnings - Perl pragma to control optional warnings
• autodie - Replace functions with ones that succeed or die with lexical scope
• diagnostics - produce verbose warning diagnostics