# Get the value for each key
for  my $key (keys %languages )
{
    print "$key --> $languages{$key}\n";
}

# Get the keys for each value
for my $value ( values %languages )
{
    for my $key ( keys %languages )
    {
        print "$key --> $value\n"
            if $languages{$key} eq $value;
    }
}
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You need to visit all the keys to do a reverse matching. In general, you also have to worry about the case where keys map to the same value.

Since you have a one-to-one mapping, you can use a pair of hashes to do efficient lookups in both directions. The downside is that you must keep the two hashes in sync.