Comparing two arrays in Perl - arrays

Question

I know this has been asked before, and I know there are functions to make this easy in Perl. But what I want is advice on my specific code. I want to go through each line of text which I've read from a file, and compare it to the same line from another file, printing them if they are different.
I've tried as many variations of this as I could think of, and none work. This specific code which I'm posting thinks every element in the array is different from the one in the other array.
use 5.18.2;
use strict;
use utf8;
printf "This program only compares two files.\n"
. "Here are the differences between "
. $ARGV[0] . " and " . $ARGV[1] . ":\n";
open FIRST_FH, '<', $ARGV[0];
chomp(my #file1 = <FIRST_FH>);
close FIRST_FH;
open SECOND_FH, '<', $ARGV[1];
chomp(my #file2 = <SECOND_FH>);
close SECOND_FH;
for(my $i=0; $i < scalar #file1; ++$i){
my $string = $file2[$i];
unless($_ =~ /$string/){
print "Difference found: #file1[$i], #file2[$i]\n";
}
}

Answer 1

use utf8; just instructs the interpreter to read your source file as UTF-8. Use the open pragma to set the default IO layers to UTF-8 (or manually specify '<:encoding(UTF-8)' as the second argument to open).
Don't use printf when print will suffice (it usually does, due to interpolation). In this particular instance, I find a heredoc to be most readable.
It's inefficient to read both files into memory. Iterate over them lazily by taking one line at a time in a while loop.
Always check if open failed and include $! in the error message. Alternatively, use autodie;, which handles this for you. Also, use lexical filehandles; they'll automatically close when they go out of scope, and won't clash with other barewords (e.g. subroutines and built-ins).
Keeping in mind these suggestions, the new code would look like:
#!/usr/bin/perl
use 5.18.2; # Implicitly loads strict
use warnings;
use open qw(:encoding(utf8) :std);
print <<"EOT";
This program only compares 2 files.
Here are the differences between
$ARGV[0] and $ARGV[1]:
EOT
open(my $file1, '<', shift) or die $!;
open(my $file2, '<', shift) or die $!;
while (my $f1_line = <$file1>, my $f2_line = <$file2>)
{
if ($f1_line ne $f2_line)
{
print $f1_line, $f2_line;
}
}
But this is still a naive algorithm; if one file has a line removed, all subsequent lines will differ between files. To properly achieve a diff-like comparison, you'll need an implementation of an algorithm that finds the longest common subsequence. Consider using the CPAN module Algorithm::Diff.

Answer 2

Why are you comparing using $_? Which you haven't defined anywhere?
my $string = $file2[$i];
unless($_ =~ /$string/){
Simply compare the lines using eq or ne:
if ( $file1[$i] ne $file2[$i] ) {
However, I would recommend that you make a lot of stylistic changes to your script, starting with doing line by line processing instead of slurping in the files. The following is how I would completely rewrite it:
use 5.18.2;
use strict;
use warnings;
use autodie;
use utf8;
my ( $file1, $file2 ) = #ARGV;
open my $fh1, '<', $file1;
open my $fh2, '<', $file2;
while ( !eof($fh1) && !eof($fh2) ) {
chomp( my $line1 = <$fh1> );
chomp( my $line2 = <$fh2> );
if ( line1 ne $line2 ) {
warn "Difference found on line $.:\n $line1\n $line2\n";
}
}
warn "Still more data in $file1\n" if !eof $fh1;
warn "Still more data in $file2\n" if !eof $fh2;