#assume that %hash has been defined
#assume that $value_to_find has been defined

foreach my $key (keys %hash)
{
    #use == instead of eq if numeric value
    if ($hash{$key} eq $value_to_find)
    {
        #$key is the key you were looking for
        print "$key has the value: $value_to_find\n";
    }
}