You've been asked several times to make some effort when asking for help, dumping code without a description of what is happening is just lazy.

Global symbol "$file1" requires explicit package name (did you forget 
+to declare "my $file1"?) at sid.pl line 5.
Can't find string terminator "END1" anywhere before EOF at sid.pl line
+ 5.
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See previous notes on use strict; use warnings;.

   open(my $f3, ">", "C:/Users/Siddharth/Desktop/two.txt")
    or die "Can't open < input.txt: $!";
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two.txt and input.txt aren't the same file, your report is wrong as you report a reading failure when you're actually trying to write a file. Take the time to read and understand the responses and working examples you've already been given.