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use strict;
use warnings;
...
$mergedHash{$_}++ for keys %file2Hash;

print "$_\n" for grep $mergedHash{$_} == 1, keys %mergedHash;

do { chomp; $file1Hash{$_}++ } for read_file $files[0];
       ^           ^            ^      ^
...
        |                              |
        |                              + - If it only appears once (in
+ either file, but not both)
        + - Print the line