#!/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;
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while(<FILE1>){
   my $value = <FILE2>;
   $hash{$_} = $value;
}
print FILE3 join("\n", map({"$_ = $hash{$_}"} keys %hash));
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That will readu lines from file handle FILE1 and use it as a key, the value being the corresponing line in FILE2. Is it that simple? Is there some more logic I need to know about? Do you really want line1 from file1 to be the key to line1 from file2? If so that above will do it (I left out the actual opening of the files, but I assume you can do that part).

Update: You should chomp $value, as L~R so astutely points out in his code above.

not safe when the files are variable length.

while(<FILE1>){
   my $value = <FILE2>;
   warn "File lengths different!", last unless defined $value;
   $hash{$_} = $value;
}
print FILE3 join("\n", map({"$_ = $hash{$_}"} keys %hash));
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holli, /regexed monk/

open( FILE1, "file1.txt" ) or die "Unable to open file1.txt\n$!\n";
open( FILE2, "file2.txt" ) or die "Unable to open file2.txt\n$!\n";

my %hash;

@hash{ map{ chomp; $_ } <FILE1> } = 
  map { chomp; $_; } <FILE2>
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similar

use File::Slurp;

my %h;
@h{map{chomp;$_}read_file("test1.txt")}=(map{chomp;$_}read_file("test2
+.txt"));
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holli, /regexed monk/