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use warnings;
use strict;
...
  my @k = unpack "C*", $k; 
  print pack "C*", map { $_^shift @k } unpack "C*", $t;
}

# make a key file, ok for demo purposes
$ perl -e 'print chr int rand 256 for 0..10000' > key.file
...
# verify
$ diff -s plain.txt deciphered.txt 
Files plain.txt and deciphered.txt are identical