I'd be keeping track of the keys when you're assigning to the definition hash. That is, keep a separate hash with duplicate keys. There is probably a more efficient way to do this, but some random puttering around while I'm waiting for my work script to finish:

#!/usr/bin/perl
use strict;
use warnings;

my (%hash, %dup_hash);
# Minor tweak to read from DATA rather than a file
while (my $line = <DATA>) {
    chomp($line);
    my ($enu, $deu) = split /\t/, $line;
    
    $hash{$enu} = $deu;

    # Keep a list of all duplicate values
    push @{$dup_hash{$deu}}, $enu;
}

for my $key (keys %hash) {
print "$key\n";
}
for my $value (values %hash) {
print "$value\n";
}

print "\nDuplicate definitions:\n";
for my $deu (keys %dup_hash) {
    if (scalar @{$dup_hash{$deu}} > 1) {
        for my $en (@{$dup_hash{$deu}}) {
            print "$deu => $en\n";
        }
        print "\n";
    }
}


__DATA__
Retire a document    Dokument deaktivieren
Remove a document from the knowledge base    Dokument aus der Knowledg
+e Base entfernen
Promote document retirement    Dokument deaktivieren
Document Expired    Dokument abgelaufen
[download]

Gives the output:

Remove a document from the knowledge base
Document Expired
Promote document retirement
Retire a document
Dokument aus der Knowledge Base entfernen
Dokument abgelaufen
Dokument deaktivieren
Dokument deaktivieren

Duplicate definitions:
Dokument deaktivieren => Retire a document
Dokument deaktivieren => Promote document retirement
[download]

Edit: Renamed some of the variables to accurately reflect their contents. Second edit Pretty much the same thing as what JavaFan has, just different syntactical approach.