use strict;
use warnings;

my %lookup = ();
open(F1, "< datei1.txt") or die $!;
while(<F1>) {
   $_ =~ s![\r\n]+$!!;
   my ($key, $val) = split /-/, $_;
   $lookup{$key} = $val;
}
close F1;

my $r = join "|", map keys %lookup;

open(F2, "< datei2.txt") or die $!;
open(F3, "> datei3.txt") or die $!;
while(<F2>) {
   s!($r)!$lookup{$1}!ge;
   print F3 $_;
}
close F3;
close F3;