MrChat,
The following code is a starting point. You need to modify it accordingly to your given situation (how to handle files of difference number of lines, duplicates, or preserving order for instance).
#!/usr/bin/perl
use strict;
use warnings;
open(FILE1, '<', 'file1.txt') or die $!;
open(FILE2, '<', 'file2.txt') or die $!;
open(FILE3, '>', 'file3.txt') or die $!;
my %hash;
while ( <FILE1> ) {
chomp;
my $val = <FILE2>;
chomp $val;
$hash{ $_ } = $val;
}
print FILE3 "$_ = $hash{ $_ }\n" for keys %hash;